perl 5.10 assertions: friend or foe?

I’ve been writing mini-articles about some of 5.10’s new features for ABE.pm, this past week. It’s been fun, and I’ve learned a few things that I didn’t know in the process. The feature that I knew the least before covering it, so far, has been assertions. Assertions let you have code that only runs if some set of assertions are enabled. It’s something like this:

sub everything_is_ok : assertion { print "Everything is okay!\n"; }

everything_is_ok;

{
  use assertions 'foo';
  everything_is_ok;
}

{
  use assertions 'foo && bar';
  everything_is_ok;
}

If I were to run this program with perl, it would print one line. If I were to run it under perl -A=foo, it would print two. If I were to run it with perl -A=foo,bar, it would print three.

Because assertions must be specified before compilation, it’s possible to optimize away the sub calls at compile time, so it’s more efficient than just checking, say, $ENV{PERL_ASSERT_FOO}.

I think it’s a swell idea, but so far a few things have left me underwhelmed.

First of all, assertions don’t prevent method calls. I’m sure that this would lead me to some profound grief in the future, when I find that a subclass really needs a subclassed assertion and I have to do more than just subclass it. It’s been mentioned that maybe this (and related woes) will be fixed before 5.10.

The whole interface seems a bit clumsy, though. I had expected it would work like this:

sub everything_is_okay : assertion(wtf) { ... }

…and then this code would only run when “wtf” exceptions were enabled. I guess the problem with this might be that sometimes you’d want it to run and sometimes not – that’s why the assertion’s power was moved to the calling block. The thing is, why isn’t the whole block compiled in or not? It would solve the problem of method calls, and it would prevent this problem:

sub check_stuff : assertion { ... }

my $code = \&check_stuff;

{
  use assertions 0; # should prevent all assertions
  $code->();        # ... but this will run anyway
}

Further, how weird is this:

{
  use assertions 'debugging';
  my $value = some_assertion;
}

If the debugging assertion isn’t on, $value is set to zero. I’d expect a compile-time death, or at least a runtime death, or at least a result of undef.

The docs for assertions aren’t great, yet, but I hope I can help. What I hope even more is that I can find a simple way to use the existing assertions hooks to write mine like this:

sub does_something_fantastic {
  ...

  assuming 'foo && bar' {
    my $object = get_object;
    $object->some_method;
    say "foo and bar must be enabled!";
  };

  ...
}

…but then again, maybe someone will just be able to point out why the implemented way is the One True Way To Do It.

Written on May 30, 2007
🐪 perl
🧑🏽‍💻 programming